As observational tools improved, astronomers saw that, like Earth, the planets rotated around tilted axes, and some shared such features as ice caps and seasons.
The ambiguities inherent in defining planets have led to much scientific controversy.
In order of increasing distance from the Sun, they are the four terrestrials, Mercury, Venus, Earth, and Mars, then the four giant planets, Jupiter, Saturn, Uranus, and Neptune.
Six of the planets are orbited by one or more natural satellites.
Planets are generally divided into two main types: large low-density giant planets, and smaller rocky terrestrials.
Under IAU definitions, there are eight planets in the Solar System.
In 2006, the International Astronomical Union (IAU) officially adopted a resolution defining planets within the Solar System.